Development of Novel Ash Hybrids

Research Issue

Our native North American ash species are highly susceptible to attack by invasive emerald ash borers (EAB). The massive devastation of ash observed as EAB has moved throughout the Midwest is in stark contrast to the situation in Asia, the region of origin of EAB. Outbreaks of EAB in Asia appear to be isolated responses to stress and do not result in devastation of the Asian ash population. Our research is based on the hypothesis that Asian ash species have evolved resistance genes and mechanisms that allow them to coexist with EAB. The long-term goal of our research is to introduce these resistance genes into the native North American ash species by producing novel ash hybrids and then performing subsequent rounds of backcrossing to recover all of the characteristics of the native North American species while maintaining EAB resistance. Such hybrids will also allow characterization of the mechanisms involved in EAB resistance through genomic and biochemical approaches. Identification of genes involved in resistance will help further an ash breeding program through the development of genetic markers that can be used for selection and screening of resistant trees.

Our Research

Since 2005, 31 different combinations of ash species have been used in controlled cross-pollination studies. Nine species were used as the maternal parent; eight as pollen donors. A total of 1,619 seeds were produced, but only three different combinations of species produced a total of 38 seedlings. The successful hybrids include crosses of F. chinensis (Chinese ash) and F. americana (white ash); F. angustifolia spp. syriaca (Syrian ash) and F. mandshurica (Manchurian ash); and F. angustifolia spp. syriaca (Syrian ash) and F. nigra (black ash). The Chinese-white ash hybrids have been confirmed based on a type of genetic fingerprinting called AFLP analysis, and efforts are ongoing to confirm the remaining seedlings as true hybrids. Not surprisingly, the successful hybridizations were between species that are closely related phylogenetically.

Another important aspect of a breeding program is the ability to select young seedlings for the desired trait, in this case resistance to EAB. Host selection and feeding bioassays are being performed to compare hybrid ash to their parent species and look for indications of EAB resistance. To date, significant differences were observed in the number of EAB landings on Chinese ash compared to two hybrids and white ash. At some of the timepoints, the two hybrids appear to be of intermediate preference to EAB–not statistically different from either parent, despite significant differences between the parents. Although only two hybrids are not enough to predict segregation patterns of the EAB host preference phenotype, the data indicate that host selection bioassays may be sufficient to detect differences among young seedlings. Differences observed using similar bioassays correlated to species-level difference observed in the field (Anulewicz et al., 2007, Pureswaran and Poland, in press) making this a useful screening technique in a hybrid breeding program. However, seedling plots will still need to be established to confirm bioassay results in field tests.

Expected Outcomes

Novel ash hybrids of select North American and Asian ash species will be developed and assessed for their EAB resistance phenotypes. Novel EAB-resistant ash hybrids can be directly marketed as street trees and for use as ornamental trees, returning ash to a profitable status in the nursery industry. In the longer term, the hybrids would be the first step in the introduction of Asian resistance genes into North American ash species through subsequent rounds of back crossing. Such an approach would allow the retention of all of the traits of the North American species along with EAB-resistance, similar to the American Chestnut Foundation’s breeding program with Chinese and American chestnut. Proteins and genes involved in EAB resistance will be identified and utilized in marker aided selection in an ash breeding program.